caa a22 4500 606198717 CHVBK 20210128100935.0 cr unu---uuuuu 210128e20150501xx s 000 0 eng 10.1007/s00348-015-1972-0 doi (NATIONALLICENCE)springer-10.1007/s00348-015-1972-0 Experimental characterization of the unsteady natural wake of the full-scale square back Ahmed body: flow bi-stability and spectral analysis [Elektronische Daten] [Raffaele Volpe, Philippe Devinant, Azeddine Kourta] In recent years, the increasing interest in reducing the aerodynamic drag of vehicles, such as station wagons, minivans or buses, has led research to focus on the characterization of square back bluff geometries. In this paper, the results of an extensive experimental campaign on the full-scale well-known body of Ahmed etal. (1984) are presented, for two height-based Reynolds numbers, $$Re_{\rm H} = 5.1 \times 10^5$$ R e H = 5.1 × 10 5 and $$7.7 \times 10^5$$ 7.7 × 10 5 . Eighty-one measurement points were used to map the base pressure field, while the wake topology was investigated by means of a series of ten 2D Particle Image Velocimetry planes. These measurements clearly show that the wake presents a bi-stable behavior, characterized by a random succession of switches between two well-defined mutually symmetric configurations, confirming the results from Grandemange etal. (J Fluid Mech 722:51-84, 2013b. doi: 10.1017/jfm.2013.83 ) for the same model. For the presented results, the timescale of this phenomenon is of the order of $$800 \, V_{\infty} / H$$ 800 V ∞ / H . The sensitivity of the bi-stability to the yaw angle was also investigated, and considerations on how to take such a behavior into account in post-processing this kind of field are given. High-frequency measurements were also carried out with four piezoelectric transducers and a synchronized two-component hot-wire. The results show a low-frequency spectral activity: peaks at $$St_{\rm H} = 0.13$$ S t H = 0.13 and 0.19, corresponding to vortex shedding modes, were found on the lateral base pressures and in the far wake, whereas a signature at $$St_{\rm H} = 0.08$$ S t H = 0.08 was visible on the vertical base centerline and in the recirculation bubble shear layer. Correlation analysis and proper orthogonal decomposition confirm the interpretation of the latter mode as the pumping of the recirculation bubble. Springer-Verlag Berlin Heidelberg, 2015 Volpe Raffaele INSA-CVL, PRISME, University of Orléans, EA 4229, 45072, Orléans, France aut Devinant Philippe INSA-CVL, PRISME, University of Orléans, EA 4229, 45072, Orléans, France aut Kourta Azeddine INSA-CVL, PRISME, University of Orléans, EA 4229, 45072, Orléans, France aut Experiments in Fluids Springer Berlin Heidelberg 56/5(2015-05-01), 1-22 0723-4864 56:5<1 2015 56 348 https://doi.org/10.1007/s00348-015-1972-0 text/html Onlinezugriff via DOI BK010053 XK010053 XK010000 Metadata rights reserved Springer special CC-BY-NC licence nationallicence 1 research-article jats NATIONALLICENCE NATIONALLICENCE NL-springer NATIONALLICENCE 856 40 https://doi.org/10.1007/s00348-015-1972-0 text/html Onlinezugriff via DOI NATIONALLICENCE 700 1- Volpe Raffaele INSA-CVL, PRISME, University of Orléans, EA 4229, 45072, Orléans, France aut NATIONALLICENCE 700 1- Devinant Philippe INSA-CVL, PRISME, University of Orléans, EA 4229, 45072, Orléans, France aut NATIONALLICENCE 700 1- Kourta Azeddine INSA-CVL, PRISME, University of Orléans, EA 4229, 45072, Orléans, France aut NATIONALLICENCE 773 0- Experiments in Fluids Springer Berlin Heidelberg 56/5(2015-05-01), 1-22 0723-4864 56:5<1 2015 56 348